Comparison of ice accumulation on simplex and duplex conductors in parallel overhead transmission lines in Iceland.

Árni Jón Elíasson – Árni Björn Jónasson – Pétur Thor Gunnlaugsson

Measuring site • Located on a ridge between two narrow

valleys

• 65 km from the shore to NNE

• Unshielded against winds from NNE

• Mostly in-cloud icing, but wet snow icing occurs too

OHTL‘s characteristics

• Fljótsdalslína 3 OHTL is fitted with simplex Austria 49.9 mm Spans 205 and 192 m

• Fljótsdalslína 4 OHTL is fitted with duplex AACSR 39.2 mm Spans 175 and 192 m Subspans 30 – 46 m between spacers

Measuring equipment

• Load cells: CAT-1 Transmission Line Monitoring System – The Valley Group, Inc.

• Energized with in-feed from closest substation

• Data transfer to database by fiber optic cable

• Online camera

Data acquired

• Data was collected over 9 year period

• 2½ winters unusable (problems in measurements or communication)

• Temperature and load readings are taken every 5 minutes

• 61 well registered icing events observed

• Up to 242 N/m for simplex and up to 163 N/m for one in duplex were observed (100:67)

Missing Data Missing Data

Wind effect

• Effect of wind on the readings was neglected in the analysis, thus the calculated ice load is a bit too high

• The effect was checked for 12 icing events (18 max-load points)

• 7 readings from the automatic weather station

• 11 readings from Egilsstaðir-airport weather station

• 10 min average wind 0 – 13.7 m/s

• Effect of wind < 1 N/m, (0.14 N/m on average).

Relationship between simplex and duplex

• Maximum ice load during icing event

• The maximum is not always at the same time for the two OHTL‘s

• In general the maximum is lower for one conductor in duplex

• Measurements are quite scattered

• Certain tendencies can be seen

• More observations in the higher load range needed to establish model

Maximum cases

• 13-28 November 2006 – 16 days – before energizing

• Temperature: Begins just below 0°C, gradually lower during 3 days down to –14 °C. Raised a little – remained low –4 °C to –9 °C .

• Duplex: Ice shedding first after about one day, occurred again a few times

• Simplex: Ice shedding did not occur until at the end of the event

• 18-21 March 2010 – 4 days

• Temperature: Begins just below 0°C, gradually lower and remains at -2 °C for a day, then lowering down to -6 °C during the day after

• Duplex: Ice shedding first after a little less than two days, but no shedding until that time

• Simplex: Ice shedding first after approximately half a day

Case 1

Case 2 Form of the accumulation graphs shows that the accumulation is similar for duplex and simplex – just shifted downwards by the ice shedding

Undisturbed ice accumulation

• The icing events where ice accumulation was undisturbed • Up to the first ice shedding in either of

the OHTL‘s

• No obvious wind disturbances during the beginning of the icing process

• Ice accumulation is 7% less for one conductor in duplex

Torsional stiffness

• Theoretical model in: C. Hardy, A. Leblond and D. Gagnon, „Theoretical Assessment of Ice Loading of Cables as a Function of their Torsional Stiffness, IWAIS XI, Montreal, June 2005

• Approximation to relate to the theoretical model:

• Precipitation at 45° angle

• Torsional stiffness for a bundle additionally involves effect of the conductor tension

• Effect of eccentric ice load on simplex and bundle of two

• Rotation of the bundle + rotation of the supspans

Comparison to theoretical model

• Simplex k*=15, duplex k*=24

• Ice density 5400 N/m3

• Going out from 50 N/m on the simplex – reduced volume = 14.9

• Reading through the diagram gives reduced volume = 21.2 for the duplex – 43.7 N/m, i.e 87% of the load on the simplex

• Going to the k*=16 for both as if the torsional stiffness was the same gives ca 46.7 N/m – 93%, i. e. 7% less for the duplex. Is the difference just due to different diameters? No effect of torsional stiffness?

Theoretical torsional stiffness models versus reality

• Precipitation angle different from 45° - less moment

• The icing does not set on the conductors as a wing mostly on one side – less moment – may be the case for the in-cloud ice

• Torsional stiffness for the bundle system not correct – difficult to estimate

Effect of ice shedding

• On average the ice shedding starts at lower ice loads for the duplex

• Ice shedding is more frequent in case of the duplex

• Duplex 75 cases observed • Simplex 55 cases observed

Simplex Duplex

Reasons for ice shedding

• More wind motions of the duplex, hence dynamic forces

• Irregularities by the spacer may sometimes give weaknesses in the ice

• Relative motions in spacer connections cause ice to break off

• Energy considerations – ambient, solar, energy transfer

• Larger weak areas for duplex due to less rotation during icing – weaker ice coat on the leeward side

Temperature and energy considerations

• The icing event left shows that ice shedding happens when ambient temperature rises above -2.5°C to -2.0°C

• Possible reasons • Ambient temperature • Solar radiation may warn up the conductors, even

through a coat of ice • Energy transfer of the OHTL’s generates heat

(6 W/m in each conductor in duplex, and 13.7 W/m in the simplex)

• If the conductors warm up the ice will at some point start melting and the bonds between ice and conductor become weak

Concluding remarks

• Undisturbed accumulation gave about 7% lower ice load on one in duplex compared with simplex in the Fljótsdalslína 3 and 4 OHTL´s • It could not be verified that this is due to difference in torsional stiffness

• Ice shedding, which lowers maximum loads for both simplex and duplex, has more effect on duplex • Is the main reason for lower loads on the duplex on average

• Ice shedding is a complex unpredictable process affected by various reasons (ambient temperature, solar radiation, heat from energy transfer, wind loads, wind induced dynamic loads, irregularities by spacers or in ice formation)

• One icing event (case 2) where duplex gets much greater ice load than simplex

Thank you for your attention !